The 'FCT157T devices are quad two-input multiplexers that select four bits of data from two sources under the control of a common data-select (S) input. The output-enable (E) input is active low. When E is high, all of the outputs (Y) are forced low, regardless of all other input conditions. Moving data from two groups of registers to four common output buses is a common use of the 'FCT157T devices. The state of S determines the particular register from which the data comes. It also can be used as a function generator. These devices are useful for implementing highly irregular logic by generating any 4 of the 16 different functions of 2 variables, with 1 variable common. The 'FCT157T devices are logic implementations of a four-pole, two-position switch, where the position of the switch is determined by the logic levels at S. These devices are fully specified for partial-power-down applications using Ioff. The Ioff circuitry disables the outputs, preventing damaging current backflow through the device when it is powered down.